Electro zinc plating solution

ABSTRACT

A cyanide-free zinc plating bath is described. Besides a zinc salt in the usual concentration range and possible brighteners, it comprises a polyhydroxy acid or salt and boric acid and/or borate in a molar ratio of between about 1(zinc):1 1/2 (polyhydroxy):3(boric) and 1:2:4, depending on the zinc content of the solution.

United States. Patent Boose et al. [4 1 Feb. 15, 1972 [54] ELECTRO ZINC PLATING SOLUTION [56] References Cited [72] Inventors: Cesar Adrianus Boose, The Hague; Maar- UNITED STATES PATENTS ten Johan Reidt, Voorburg, both of N h l d 3,298,938 1/1967 Shibasaki ..204/55 R [73] Assignee: Nederlandse Organisatie Voor Toegepast- FOREIGN PATENTS OR APPLICATIONS Natuurweten-Schappeliik Onderzoek Ten Behoeve v Nijverheid Handel en v l,688 12/1902 Great Blltaln ..204/55 keer, The Hague, Netherlands Primary Exammer-G. L. Kaplan I Filed! 1970 Attorney-Brumbaugh, Graves, Donohue and Raymond 7 7 [21] Appl No l ,9 8 ABS CT [30] Foreign Application P i it D t A cyanide-free zinc plating bath is described. Besides a zinc salt in the usual concentration range and possible brighteners, Mar. 1], Netherlands 1 it comprises a olyhydroxy acid or Salt and acid and/ r borate in a molar ratio of between about l(zinc): l r(polyhydr [52] US. Cl. ..204/55 R oxy);3(boric) and 1 2:4 depending on the zinc content f the [51] Int. Cl ..C23b 5/10, C23b 5/12, C23b 5/46 Solution [58] Field of Search ..204/55 Y, 55 R, 43, 44, 114

6 Claims, No Drawings With these conventional protracted inhomogeneities. A

gelatin, 3,4-methylene-dioxy-benzaldehyde (piperonal), 4- hydroxy-3-methoxy-benzaldehyde (vanillin), 3,4-dimethoxybenzaldehyde (veratraldehyde), protein decomposition products, and the like.

All such conventional additives can also be used within the scope of the present invention.

Prior to the electro zinc plating process taking place, the metal to be zinc plated is usually subjected to such pretreatments as degreasing. A common pretreatment is for instance degreasing in organic solvents, followed by degreasing in an alkaline medium, which may be carried out electrolytically, if necessary. After rinsing with water and pickling with hydrochloric acid, there is another rinsing with water. This is followed by the zinc plating process. After this, the metal is usually passivated. Examples of conventional passivating liquids are (all per litre ofsolution):

CrO 30 g. H Po, (pure) l cm. HCl (cone) 5 cm. HNO (conc.) 5 cm. H 50, (concJ 5 cm. or

Na,Cr O, 200 g. H 80, (conc.) 6 cm.

methods, the temperature is somewhere between 10 and 80 C. and the dipping time between I5 and I seconds. The material is passivated to give extra protection against corrosion and to provide a basis for a finish such as the application of a lacquer.

These preliminary and subsequent treatments are generally known techniques, and they can also be applied within the scope of the present invention.

There are objections against using cyanide solutions. It is particularly the high toxicity which gives rise to serious problems in removing spent solutions. Attempts have therefore been made for a very long time to use noncyanide solutions but in electro zinc plating cyanide solutions nevertheless still constitute a majority. Sometimes zincate solutions, which are based on the solubility of zinc salt in an alkaline medium, are used. Zincate solutions are expensive, however, and extremely sensitive to metal impurities.

Electro zinc plating is carried out mainly either in revolving drums or in stationary containers. in the first case the solution is automatically shaken, which is essential to prevent stationary solution can be agitated for instance by means of air.

I The current density on the objects to be zinc plated is also according to the present method between about 0.1 and l0 amperes per dmI; it is usually between about 3 and 6 amperes per dm.

The temperature is not so very important for the present method. The solution is an aqueous liquid, which means that it always remains above its solidification point and, reasons, below its boiling point. In practice, the temperature will be somewhere between 20 and 80 C. It is true that at a higher temperature the electric conductivity of the solution increases and the efficiency improves, but at a lower temperature the brightness of the galvanized product is generally slightly better. The choice is therefore a matter of practice rather than theory.

Solutions are known which have a high or a low zinc content, each having its own specific advantages. Exactly the Two terms are used in electro zinc plating. "Variance is used to indicate the following.

Assuming an object with many projecting and recessed corners, of which a common example is a metal screw bolt with its screw thread and the slot in its head for the screwdriver.

Ideally, the applied zinc coat is equally thick everywhere; the I degree of difference is referred to as variance.

Cover is used to indicate the following.

Assuming an iron cup has to be zinc plated. The cup is suspended in the solution and it is also filled with this liquid. ln electroplating, ideally the inside bottom of the cup is also entirely covered with zinc; this effect is referred to as cover.

It is known that variance and cover are very much dependent on the composition of the solution.

it will be clear from the foregoing that all kinds of solution components and electroplating conditions are known, and all these conventional matters apply likewise to the present invention. its essence is that the electroplating solution per litre of the bath must also contain 0.05 to 0.8 gram-atom zinc, a polyhydroxy acid or a salt thereof, as well as boric acid and/or borate. The molar ratio of zinc content and the polyhydroxy anion should be between about 1:1.3 and 1:22; the molar ratio of polyhydroxy anion to boric anion between about 1:1.8 and 1:22

For the effect it makes no difference whatsoever whether polyhydroxy acid or a salt thereof is used and it is equally unimportant whether boric acid or borate is used. The choice is determined exclusively by factors beyond the scope of the invention, such as price, availability, keeping properties, and the like. lt now appears advantageous to use sodium gluconate or sodium heptonate and boric acid, but this has nothing to do with the essence of the invention.

lt is known that sometimes a low and sometimes a high zinc content has to be used. in practice a low zinc content is considered to exist in a solution with approximately 5 to 25 grams of zinc per litre, and a high zinc content in a solution with approximately 25 to 50 grams of zinc per litre.

For a low zinc content the three-component system, which forms the essence of the invention, appears to be best if made up of approximately 2 gram-molecules polyhydroxy acid or a salt thereof, and approximately 4 gram-molecules boric acid and/or borate per gram-atom zinc 10 percent deviation possible).

In solutions with a high zinc content, these ratios per gramatom zinc are about at 1V2 gram-molecule polyhydroxy acid or a salt thereof and 3 gram-molecules boric acid or borate (10 percent deviation possible).

At about 25 grams of zinc per litre ratios intermediary between the above-mentioned ratios may be used.

It has already been pointed out in the foregoing that the question whether polyhydroxy acids or salts thereof, or boric acid or borate are used, has nothing to do with the essence of the invention. This is all the more important because it is desirable to keep the pH of the solution between certain limits, which means in practice that free acids or free lyes are added. in this case dissociation equilibria occur, which make a difference between for instance boric acid and borate irrelevant. At all times the pH must be above 5; below this good results cannot be achieved with certainty. The upper limit depends on the composition of the solution. Particularly on the substance, mostly an inorganic salt such as sodium sulphate, which is present in the solution to increase the electric conductivity. With sulphates, the pH must not be raised beyond approximately 8, but with chlorides the limit may be higher. By means of a few tests, a specialist should be able to find the correct pH, that is the pH at which, given the composition of the solution, the electro zinc plating process proceeds at an optimum level.

The zinc compound, which is always necessary in electro zinc plating baths, is usually and also according to the present method zinc sulphate or zinc chloride, but in view of the required pH it still makes no difference whether zinc sulphate and, for instance, sulphuric acid are added as pH controlling agents or another zinc compound such as ZnO, which in this case with H 80. again brings zinc and sulphate ions into the solution anyway.

Ammonium ions should be avoided; they give less favorable results.

EXAMPLE] A drum 30 cm. long and cm. in diameter, with perforations totaling 6.7 percent, fitted with flexible contacts, contained 300 screw bolts with a total surface of 75 dm. The drum was placed in a zinc solution of 185 litres, which had the following composition per litre ofsolution:

zinc l6.5 grams (as ZnSO -7H,O

sodium gluconatc l l0 grams boric acid 70 grams anhydrous sodium sulphate I00 grams sodium hydroxide [3 grams bcnzaldchydc 0.2 gram water to make 1 liter The pH ofthe solution was 6.8. During galvanizing, the temperature remained between 50 and 60 C. The electroplating time was I hour at a current density of l ampere per dm. After this, the bolts were rinsed with water and put in a passivating solution, and finished. The efficiency was 75 percent.

As the benzaldehyde tends to become exhausted during electroplating, aldehyde was added after each drum charge in such a way as to maintain the aldehyde content at about 2 grams per litre.

EXAMPLE ll.

Example I was repeated with a solution of the following composition per litre ofsolution:

yinc 18.7 grams 9L5 grams ISO grams The pH was kept at 6.8, the temperature was 45 C.

The charge was 15 kg. carriage bolts with a total surface of dmf"; the electroplating time was 35 minutes, at a current density of 0.85 amperes per dm. The efficiency was 78 percent and the coat thickness of the zinc after passivating was 5.2 microns.

A remarkable advantage of these solutions is that with an amount of brightener (i.e., benzaldehyde) less than normal a gloss was obtained which was brighter than normal.

The cover of the objects zinc plated in accordance with the invention is excellent, and the throwing power of the solutions in accordance with the invention is good.

We claim:

1. An aqueous electroplating solution comprising about 0.05 to 0.8 gram-atom zinc per litre, polyhydroxy carboxylic acid or a salt thereof, and boric acid or borate, the molar ratio of zinc to polyhydroxy carboxylic acid or salt being from about 121.3 to about 1:2.2 and the molar ratio of polyhydroxy carboxylic acid or salt to boric acid or borate being from about 121.8 to about 122.2, the pH of the solution being at least about 5, and the solution being substantially free ofammonia.

2. An aqueous electroplating solution according to claim 1 comprising between 5 and 25 grams per litre zinc, about 2 gram-molecules polyhydroxy carboxylic acid or salt thereof per gram-atom zinc, and about 4 gram-molecules boric acid or borate per gram-atom zinc. I

3. An aqueous electroplating solution according to claim 1 comprising between 25 and 50 grams per litre zinc, about 1.5 gram-molecules polyhydroxy carboxylic acid or salt thereof per gram-atom zinc, and about 3 gram-molecules boric acid or borate per gram-atom zinc.

4. An aqueous electroplating solution according to claim 1 containing from about 1.8 to 2.2 parts boric acid per part polyhydroxy carboxylic acid or salt thereof.

5. An aqueous electroplating solution according to claim 1 wherein the polyhydroxy carboxylic acid or 'salt thereof is sodium gluconate.

6. An aqueous electroplating solution according to claim 1 wherein the polyhydroxy carboxylic acid or salt thereof is sodium heptonate.

7 Patent No.

zgz gg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,642,591 Dated February 15 1972 Inventor(s) Cesar Adrianus Boose and Maarten Johan Reidt It is certified that error appears the above-identified patent and that said Letters Patent are hereby corrected as shown below:

First page, .Item [73] line 3, after "Nijverheid" insert a comma. Column 1, line 10, "most" should be --mostly-. Column 3, line 45, "18.7 grams" should be -28.7 grams--.

Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. ROBERT GOT'I'SCHALK Attesting Officer Commissioner of Patents 

2. An aqueous electroplating solution according to claim 1 comprising between 5 and 25 grams per litre zinc, about 2 gram-molecules polyhydroxy carboxylic acid or salt thereof per gram-atom zinc, and about 4 gram-molecules boric acid or borate per gram-atom zinc.
 3. An aqueous electroplating solution according to claim 1 comprising between 25 and 50 grams per litre zinc, about 1.5 gram-molecules polyhydroxy carboxylic acid or salt thereof per gram-atom zinc, and about 3 gram-molecules boric acid or borate per gram-atom zinc.
 4. An aqueous electroplating solution according to claim 1 containing from about 1.8 to 2.2 parts boric acid per part polyhydroxy carboxylic acid or salt thereof.
 5. An aqueous electroplating solution according to claim 1 wherein the polyhydroxy carboxylic acid or salt thereof is sodium gluconate.
 6. An aqueous electroplating solution according to claim 1 wherein the polyhydroxy carboxylic acid or salt thereof is sodium heptonate. 